Many optical instruments used for localized optical metrology measurements of substrates including film thickness, critical dimensions (CD), overlay instruments, and the like, use a beam of light that impinges on the surface of a sample. These technologies are often used to perform measurements on a series of sample targets, or target arrays on the substrate of interest in order to take these metrology measurements. A key consideration for metrology systems used in production environments, such as semiconductor wafer fabrication is throughput, which refers to the number of samples that can be scanned per unit time. The throughput depends on the number of targets in a sample, the time to acquire each target, the time for measurement at each target and the time to move from one target to the next. The combined time for movement, acquisition and measurement is sometimes referred to as move-acquire-measure (MAM) time. It is desirable to decrease the MAM time in order to increase the throughput or to allow more targets to be measured without detrimentally affecting throughput.
Currently the most popular technique for implementing these metrology measurements include physically moving the substrate under the optical beam of light. These small, localized moves, on the order of 10-50 micrometers of translation, may be accomplished by using conventional stepping stages. However, the movement times are restricted to minimum response times of the stepping stages. These are typically in the neighborhood of 50 milliseconds due to mechanical and inertial limitations.
Another technique implemented for these metrology measurements includes the use of scanning mirrors as the active element of the scanning tool. In this technique, the sample remains more or less fixed and the mirror scans the incident beam from one target location to another. This approach, however, has several clear disadvantages. In particular, for very small scan translations, e.g., on the order of 10-50 micrometers, the angle adjustment required for these scanning mirrors is less than 1 milliradian. Therefore the intrinsic accuracy for repeatability and cross-axis motion is severely limited.
It would be desirable to be able to reduce the target-to-target, or cell-to-cell stepping times down to less then 5 milliseconds. By decreasing the stepping-time, one can obtain a large improvement in the overall MAM time of the measurement sequence. It is within this context that embodiments of the present invention arise.